1. Field of the Invention
The present invention relates generally to an exhaust gas recirculation (EGR) control system of a type which comprises an EGR passageway having a restriction formed upstream of the EGR control valve to define a chamber between the restriction and the EGR control valve and particularly to an EGR control system of this type in which the EGR control valve is operated to reduce and increase the exhaust gas pressure in the chamber between the restriction and the EGR control valve in accordance with increases and decreases in the venturi vacuum, respectively.
2. Description of the Prior Art
As is well known in the art, an exhaust gas recirculation (EGR) control system serves to reduce the production of nitrogen oxides (NOx) in combustion of an internal combustion engine by controlling the maximum combustion temperature below a certain level by recirculating or feeding into air drawn by the engine exhaust gas emitted from the engine. Accordingly, it is necessary to control the flow of recirculated engine exhaust gas with enough consideration for the operating performance or driveability and the fuel consumption of the engine.
It is usually desirable to maintain at a predetermined or constant valve the EGR ratio, that is, the ratio of the recirculated exhaust gas flow to the flow of air taken into the engine. As an expedient for attaining this purpose, there is proposed an EGR control system of a back pressure proportioning type as shown in FIG. 1 of the accompanying drawings. This conventional EGR control system comprises an EGR passageway 1 formed therein with a restriction or orifice 2 for controlling the recirculated exhaust gas flow, an EGR control valve 3 disposed in the EGR passageway 1 downstream of the restriction 2, and a diaphragm unit including a flexible diaphragm 4 which is operatively connected to the EGR control valve 3 and has on a side thereof a fluid chamber 5 fed with a suction vacuum. The EGR passageway 1 has a chamber 6 defined between the restriction 2 and the EGR control valve 3. A pressure converting valve 7 is provided for controlling the flow of atmospheric air admitted for diluting the suction vacuum fed into the fluid chamber 5 and includes a flexible diaphragm 8 operatively connected to the valve 7 and having on a side thereof a fluid chamber 9 communicating with the chamber 6 of the EGR passageway 1. The valve 7 is operated in accordance with a pressure P.sub.o in the chamber 6 and controls the suction vacuum in the fluid chamber 5 to a value related to the pressure P.sub.o. The degree of opening of the EGR control valve 3 is feedback controlled by the suction vacuum in the fluid chamber 5 to maintain the pressure P.sub.o in the chamber 6 constant during most of engine operations. As a result, the recirculated exhaust gas flow is represented as a function of the pressure P of engine exhaust gas in the EGR passageway 1 upstream of the restriction 2 which pressure P is about proportional to the square of the engine taken air flow. Accordingly, the recirculated exhaust gas flow is controlled to an about constant ratio to the engine intake air flow.
When the recirculated exhaust gas flow is a function of the pressure P of exhaust gas in the EGR passageway 1 upstream of the orifice 2 in this manner, the recirculated exhaust gas flow is very easily affected by variations in the exhaust gas pressure P. Accordingly, when the exhaust gas pressure P is not proportional to the square of the engine taken air flow, the reliability on the control of the recirculated exhaust gas flow which is effected by the conventional EGR control system is considerably reduced.
The exhaust gas pressure is often varied independently of the engine taken air flow by parameters such as the injection of secondary air into the exhaust gas system, the temperature of the engine exhaust gas, the flow resistance of the exhaust gas passageway and so on. Accordingly, the exhaust gas pressure is highly unreliable as compared with a carburetor venturi vacuum as a function of the engine taken air flow.
Furthermore, since the exhaust gas pressure P does not have such a large absolute value and is vaired over a fairly wide range, the conventional EGR control system has been unable to control the EGR ratio to a constant or predetermined value. As a result, the conventional EGR control system has been apt to unsatisfactorily reduce the production of nitrogen oxides and to render the driveability of the engine unstable.
On the other hand, a conventional EGR control system has recirculated engine exhaust gas at a similar predetermined EGR ratio throughout all engine operating conditions. As a result, under high speed and low load engine operating condition in which the production of nitrogen oxides (NOx) is relatively small, the recirculated exhaust gas flow has become excessive to degrade the driveability and the fuel consumption i.e. fuel economy of the engine.
It is, therefore, an object of the invention to provide an EGR control system improved to comprise means for varying the pressure in the chamber interposed between the restriction and the EGR control valve in accordance with variations in a venturi vacuum in the engine intake passageway to prevent the recirculated exhaust gas flow from being greatly affected by variations in the exhaust gas pressure in the EGR passageway upstream of the chamber and to render the recirculated exhaust gas flow dependent on the pressure differential between the EGR passageway upstream of the orifice and the chamber between the orifice and the EGR control valve so that the recirculated exhaust gas flow is accurately controlled.
It is a further object of the invention to provide an EGR control system improved to comprise means for reducing the EGR ratio to prevent the driveability and the fuel consumption of the engine from being degraded during high speed and low load engine operating condition.
These objects are attained by operating the EGR control valve in accordance with the difference between the pressure in the EGR passageway between the restriction and the EGR control valve and the venturi vacuum, or by providing a control valve for controlling in accordance with the difference between, the atmospheric pressure, the pressure in the EGR passageway between the restriction and the EGR control valve, and the venturi vacuum, the flow of atmospheric air admitted for diluting the suction vacuum for operating the EGR control valve and additionally by providing a check valve for admitting in response to a suction vacuum increased above a predetermined value the suction vacuum into the atmospheric pressure for operating the control valve.